The present invention relates to a method for manufacturing a semiconductor device which includes a non-single crystalline semiconductor layer formed on a substrate, for example, an insulating substrate such as glass. In particular, the semiconductor device is of an insulated gate type such as a thin film transistor (TFT) or thin film diode (TFD). The present invention is further related to a manufacture of a thin film integrated circuit utilizing the TFT or TFD, specifically a thin film integrated circuit for an active matrix liquid crystal display device.
In recent years, active matrix liquid crystal device and an image sensor have been developed which utilize a semiconductor device having TFTs formed on an insulating substrate such as glass for driving pixels or as a peripheral circuit.
Silicon in the form of a thin film is most generally used for forming TFTs of these semiconductor devices. Generally, thin film silicon semiconductors are classified to (a) amorphous silicon and (b) crystalline silicon. While amorphous silicon is easy to form at a relatively low temperature, electrical characteristics of the amorphous silicon is inferior to crystalline silicon.
In usual, when manufacturing TFTs with a silicon film, a pattering of the silicon film has been used to isolate each element from one another. A photoresist has been used to pattern the silicon film as a mask.
On the other hand, it has been strongly required to avoid any contamination of silicon during the fabrication process of TFTs as much as possible since an active semiconductor region of a TFT is very sensitive to impurities. As a source of a contamination, the air and the handling with human hands are known. However, this problem has not been completely solved yet.
It is an object of the present invention to provide a semiconductor device such as a TFT or image sensor having a high quality by solving the foregoing problems.
It is another object of the present invention to provide a semiconductor device having a semiconductor film with an improved crystallinity.
The inventors of the present invention first recognized that a photoresist which is used for patterning a semiconductor layer is one of the sources of the contamination causing properties of the TFTs degrade. Based on this recognition, the primary feature of the invention is to provide a protective film between the photoresist and the semiconductor layer in order to prevent the impurity elements contained in the photoresist from invading into the semiconductor layer.